NASA's Space Shuttle is the spacecraft currently used by the U.S. for its human spaceflight missions, scheduled for retirement in 2010. Five space-worthy orbiters were built: Columbia, Challenger, Discovery, Atlantis, and Endeavour, the program has an estimated cost of US$170 billion. At times, the orbiter itself is referred to as the space shuttle. Technically, this is incorrect, the actual "Space Transportation System" is the combination of the orbiter (OV), the external tank (ET), and the two solid rocket boosters (SRBs). Combined, these are referred to as the "Stack"; assembled in the Vehicle Assembly Building originally built for the Saturn V rocket. Music: Apollo 13 OST.